This invention relates generally to wireless communications systems and, more specifically, to a system and method for improving the signal to noise ratio of Code Division Multiple Access (CDMA) communications by filtering colored noise.
Spread spectrum communication techniques allow communicating users to operate in noisy radio frequency (RF) spectrums, and are especially effective against narrow-band interferers. Spread spectrum communications can be effected at relatively low power spectral densities, and multiple users can share the same frequency spectrum. Further, receivers can be designed to protect against multipath. These system characteristics encouraged early development of the technology by the military.
Direct Sequence systems spread a digital stream of information, typically in a phase shift keyed modulation format, with a PN code generator, to modulate a carrier signal. The pseudonoise sequence of the PN code generator is periodic, and the spread signal can be despread in a receiver with a matching PN code. Direct Sequence systems have excellent immunity to noise. The PN codes used typically permit a large number of users to share the spectrum, with a minimum of correlation between the user""s PN codes. However, Direct Sequence system require large RF bandwidths and long acquisition times.
The IS-95 standard defines key features of the so-called second generation code division multiple access (CDMA) communication system, a type of Direct Sequence spread spectrum modulation. To help solve the problem of long acquisition time, the IS-95 signal uses a pilot channel. Each base station transmits a pilot channel message spread with PN codes known to all the mobile stations. The PN code is made up a series of phase shifted binary symbols called chips. The PN period is 32,768 chips and the PN chip rate is 1.2288 Megahertz (MHz). The digital stream of information that is spread by the PN code is also known to the mobile stations. Because there is no ambiguity in the demodulated message, the timing characteristics of the PN code, down to the chip phase, as well as the QPSK modulation phase are known to the mobile station receiver.
The IS-95 system communicates information from the base station to the mobile stations through a series of traffic channels. These traffic channels are transmit and receive information, i.e. digitized audio signals, spread with a traffic channel PN code, unique to each mobile station. Using this precise timing and phase information derived from the pilot channel, the mobile station is able to acquire a setup channel, and eventually, the overall System Time. With this System Time, the mobile station is able to differentiate between base stations and synchronize the demodulation circuitry with sufficient accuracy to recover the received traffic channel message.
A third generation, wideband CDMA (W-CDMA) system is in development as described in xe2x80x9cWideband-CDMA Radio Control Techniques for Third Generation Mobile Communication Systemsxe2x80x9d, written by Onoe et al., IEEE 47th Vehicular Technology Conference Proceedings, May 1997, that may have global applications. Instead of a pilot channel, the W-CDMA system has a broadcast, or perch channel. Each timeslot, or slot of the broadcast channel consists of a series of time multiplexed symbols. A long code masked, or special timing symbol segment uses just a short code to spread one symbol of known information. This segment allows a mobile station to acquire system timing information immediately after turn-on. The pilot, or reference symbols are similar to the IS-95 pilot channel. In one proposal, 4 reference symbols, with each symbol being 2 bits, are spread with a long code and a short code. The reference symbol information and the short code are known by the mobile stations. The long code is unique to each base station, so that timing information is refined, once the long code is known (the base station is identified). Therefore, according to some proposals, 5 symbols in the slot would be dedicated to the mobile station acquiring timing information. Further, both the long and short codes spread 5 symbols of data during each slot. Since information is not predetermined for the data symbols, precise timing information cannot be accurately recovered, as with the other two kinds of (timing) symbols. Other combinations of reference, special timing, and data symbols are also possible.
The W-CDMA system also includes several traffic channels to communicate information, such as a digitized voice or data. The traffic channel predominately includes information, but may also include a reference symbol segment. For example, at a data rate of 32 kilosymbols per second (ksps), a slot could include 4 pilot symbols and 16 information symbols. Precise timing information can be derived during the reference symbols segment of the traffic channel message, but not during the information segments.
The W-CDMA system, or any spread spectrum system, operates best by minimizing the transmitted power of the users, within the constraints of maintaining a fixed bit error rate (BER). Lower spectral power densities permit additional users to be added to the system, or an increase in the signal to noise ratio of received messages. Each mobile station is likely to receive more than one traffic channel from a base station, with each traffic channel being unique to a mobile station. That is, each base station is capable of transmitting hundreds of different traffic channels, the exact number is dependent on the traffic channel data rates. However, each base station transmits only a few, perhaps only one, broadcast channels that are used by all the receiving mobile stations. It is advantageous for the system that the base stations transmit the shared broadcast channels at a higher power level than the mobile station specific traffic channels. For this reason, the broadcast channel power is maintained at a relatively high level, while the traffic channel levels are continually monitored and adjusted to keep the transmitted power levels only as large as necessary to reasonably enable communication between the base station and the mobile.
Regardless of whether the broadcast channel power can be minimized, communications to others receivers in the system, and the broadcast channel, especially the long code masked pilot symbols, all add energy to the transmission spectrum which appears as noise to any specific receiver attempting to recover traffic channel information. Since this colored noise largely comes from a common transmitter, it is not uniformly or randomly spread across the spectrum, as is white noise.
Dent et al., U.S. Pat. No. 5,572,552, discloses a method of maximizing the SNR and canceling inter-chip interference, but it is computationally intensive. That is, it relies on subsystems not existing currently in receivers. The use of a forward link receiver is discussed using the same tap locations as a RAKE receiver. However, little mention is made of how those paths are computed. Neither is a means given for integrating such a filter with a conventional baseband receiver, such as in IS-95, or that used in the ETSI/ARIB (European Telecommunications Standards Institute/Association of Radio Industries and Businesses) proposals for IMT-2000 (International Mobile Telecommunications). No method for subtractive cancellation of a pilot signal, which on the forward link, is often relatively strong. Further, no mention of the use of multiple pilot signals or slotted pilot signals such as those proposed for third generation systems.
Jamal, et al., U.S. Pat. No. 5,727,032, discusses a least mean square (LMS) algorithm for estimating channel coefficients. This method is well known to converge to channel estimates faster, as the LMS algorithm""s convergence depends on the ratio of the covariance matrix""s eigenvalues. It also does not appear to address the colored noise problem, focusing on using this algorithm solely for channel impulse response estimation.
Kowalski et al, Ser. No. 09/048,240, entitled xe2x80x9cPilot Aided, Time-Varying Finite Impulse Response, Adaptive Channel Matching Receiving System and Methodxe2x80x9d, filed Mar. 25, 1998, and assigned to the same assignees as the instant application, discloses the use of finite impulse response (FIR) filters used to maximize the signal to noise ratio of a multipathed signal by combining delayed signals before the process of correlation.
A. Papoulis (Signal Analysis, McGraw-Hill, 327-328, 1977) suggests the use of approximations to matched filter receivers. However, the combination of such a receiver with channel estimation in colored noise, and with complex values, and the application to DS-CDMA handset receivers is novel.
It would be advantageous if RAKE receiver channel estimates could be calculated to maximize the signal to noise ratio of CDMA traffic channel communications in the presence of colored noise.
It would be advantageous if the tracking and searching algorithms already present in IS-95, and 3rd generation CDMA systems could be used to minimize the detrimental effects of colored noise.
It would be advantageous if pilot symbols could be eliminated from the received communications, for the purpose of recovering traffic channel information, increasing the effective signal to noise ratio.
Accordingly, in a code division multiple access (CDMA) communication system including at least one base station transmitting information to a mobile station, a method has been provided for removing colored noise from received communications. The method comprising the steps of:
a) receiving communications including information, colored noise, and white noise;
b) in response to the communications received in Step a), generating a pre-whitening filter to reduce the colored noise; and
c) performing channel weighting and demodulation of the received communications, filtered in Step b), to recover the transmitted information. In this manner, the received communications are selectively emphasized to compensate for the colored noise.
CDMA communications are received along a plurality of transmission paths with corresponding path delays. Step a) includes resolving the timing of the received communications associated with each transmission path. Then, Step b) includes the sub-steps of:
1) forming a functional for each transmission path based on the received noise and the timing of each transmission path resolved in Step a); and
2) in response to each functional formed in Step b)1), determining the pre-whitening filter for the colored noise of each corresponding transmission path.
Specifically, Step b)1) includes the functional as follows:
J=xcex1HRxcex1xe2x88x92xcex[xcex1Hfxe2x88x92y0];
in which xcex1 represents the pre-whitening filter;
in which ( )H denotes a conjugate transpose;
in which R is the noise covariance matrix;
in which J is the functional value;
in which y0 is a constant;
in which f is the received communication; and
in which xcex is a non-zero LaGrange multiplier.
CDMA transmissions typically include a pilot signal. Step a) includes receiving the pilot symbol as part of the received communications. A further step follows Step a) of:
a1) in response to receiving the pilot symbol in Step a), creating a pilot symbol replica;
Then, Step b) includes using the pilot symbol replica to cancel the received pilot signal from received communications and colored noise, whereby the pre-whitening filter is optimized, and Step c) includes subtracting the pilot symbol replica from the received communications to provide a channel estimate without the effects of the pilot symbol, whereby the pilot symbol is filtered as colored noise.
In a code division multiple access (CDMA) communication system including at least one base station transmitting information to a mobile station, a receiver for maximizing the signal to noise ratio in the presence of colored noise is also provided. The receiver comprises an autocovariance estimator to receive the transmitted communications including information, colored noise, and white noise, and to provide a covariance matrix of noise statistics. A pre-whitening filter having an input operatively connected to the output of the autocovariance estimator and provides the optimal estimation vector in response to the covariance matrix. A RAKE receiver accepts the transmitted communications including information, white noise, and colored noise, as well as the optimal estimation vector. The RAKE receiver uses the optimum estimation vector to provide despread received information at an output, whereby the probability of receiving information is improved.
In some aspects of the invention, the CDMA system includes transmission of a pilot symbol to aid in synchronization and timing. A pilot symbol replica generator accepts information to determine the occurrence of the pilot symbols and provides a pilot symbol replica at the same time that an actual pilot symbol is received. A subtractor circuit accepts the received signal and the pilot symbol replica. The received signal without the pilot symbol is then input into the autocovariance estimator. The autocovariance estimator is able to provide a vector in the matrix of noise statistics which effectively filters colored noise from the received signal.